I am no expert. I am currently dealing with betrayal in my marriage with regard to sexual intimacy.
I do not understand the comment about "how does he even know about this stuff". I would think that by the time you are in your seventies you would know about most stuff, whether you choose to engage in it or not. On the other hand, I do not think that missionary style is the only acceptable style of sex that is intimate or not sinful. Intimacy is what the two of you make it, isn't it? It is two people who feel safe sharing what they BOTH want to share.
As a woman who has been married 35 years, it sounds to me like your husband is asking you to do things that are new and things you are not comfortable with. You are not crazy. You have been with this man for 40 years. Something about it has you feeling uncomfortable - perhaps it is new behavior, perhaps he is shaming you for not wanting to do it. Trust your instincts and respect your body and soul. Personally, I believe the Holy Spirit is warning you that something is not right. Forget the concept of sinful. AThis seems to be confusing you. The Bible tells us that all we need to know, the Holy Spirit will reveal. Secularly, people say to trust your gut. If it feels wrong to you, it is wrong. But let's be clear, doing something new or different is not wrong so long as you both want to try it. This does not seem to be the case here.
If I had to guess, I would say he has been looking at a lot of inappropriate content. But that is a guess based on my own recent revelations and what looks to be some familiar behavior. Wishing you strength.